1. Field of the Invention
The present invention relates to a bookbinding apparatus and image forming system for binding a sheet bundle after coating the back of the sheet bundle with adhesive.
2. Description of Related Art
There has been a spread of on-demand printing system wherein an image is formed on paper, and a plurality of sheets of the paper with image formed thereon are made in a bundle, which is then bound and formed in a booklet. Such a printing system is often produced by a bookbinding apparatus wherein a hot melt blue is applied onto the back of the sheet bundle made by stacking the sheets of paper having an image formed thereon, and the sheet bundle is then bound to produce a booklet.
The adhesive applied to the back of a sheet bundle is a solid pellet at the normal temperature, and is dissolved and liquefied by being heated in the adhesive container of the coating apparatus. Then the adhesive is coated.
The Japanese Unexamined Patent Application Publication No. 2007-62202, Japanese Unexamined Patent Application Publication No. 2007-57580 and Japanese Unexamined Patent Application Publication No. 2006-62350 propose the method of turning off the adhesive heating heater when the coating apparatus is not used for coating, and the method of maintaining the adhesive temperature below the temperature required for coating, from the viewpoint of power saving to minimize the power consumption resulting from adhesive heating, and from the viewpoint of protecting against odor produced by adhesive heating.
The bookbinding apparatus in the print on-demand system is not working when the whole system is in the standby mode. The apparatus is working as a whole system, but is regarded as not working during execution of a job other than bookbinding.
Thus, from the viewpoint of power saving and protection against odor, it is more effective to turn off the heater for heating the adhesive, or to keep the adhesive temperature lower than the coating temperature, when the bookbinding apparatus is not working.
Such an adhesive temperature control is also effective to protect against deterioration of the adhesive.
To be more specific, if the adhesive is placed under the conditions of high temperature for a long time, discoloration or degradation of adhesive force will proceed. When the bookbinding apparatus is not working, adhesive temperature must be reduced.
As described above, when the bookbinding apparatus is not working, control is provided in such a way that the adhesive heating heater is turned off, or the adhesive temperature is set at the low temperature which is higher than the normal temperature but is lower than the coating temperature. The heater is turned on by the bookbinding startup signal to raise the adhesive temperature, whereby coating is started.
However, in the control during the shut-off period of the bookbinding apparatus and startup control, the following problems have been found to occur:
As described with reference to the Japanese Unexamined Patent Application Publication No. 2007-62202, the adhesive temperature in the adhesive container is managed by the control using the temperature sensor. The operation of the bookbinding apparatus starts in response to the signal of the temperature sensor, which indicates that the temperature has reached the level that allows the adhesive to be coated.
The temperature sensor for detecting the adhesive temperature in the adhesive container is arranged at the center of the adhesive container so as to detect the temperature close to that of the adhesive drawn up by the coating roller located at the center of the adhesive container.
Thus, the temperature sensor detects the adhesive temperature at the center of the adhesive container. However, since the adhesive has a high degree of viscosity, there is little convection of adhesive in the adhesive container. If the molten adhesive is left at rest, a difference occurs between the adhesive temperature at the center and that on the periphery.
FIG. 4 shows such a temperature difference. The adhesive temperature at the center CA is higher than that on the periphery PA.
The temperature sensor 57 placed at the center CA detects the high temperature at the center CA. In the presence of such a temperature difference, the coating roller 51 rotates to start coating operation.
As described above, such control as temperature management is performed according to the temperature detected by the temperature sensor 57. When the high temperature at the center CA is detected and coating of the adhesive has started, the sufficiently molten adhesive is coated in the form of a lump. This may result in uneven coating in some cases.